1. Field of the Invention
This invention relates to a means to minimize the time to cool down a mass to cryogenic temperature using a refrigerator that operates on a Brayton or GM cycle.
2. Background Information
Most cryogenic refrigerators are designed to provide refrigeration at a low temperature over a long period, and system simplicity is given priority over efficiency during cool down. Most expanders and compressors are designed to operate at constant speed and most systems have a fixed charge of gas, usually helium. The mass flow rate through the expander is proportional to the density of the gas, thus when the expander is running warm it has a much lower flow rate than when it is cold. The compressor is sized to provide the flow rate that is needed when the unit is cold and the system is usually designed with an internal pressure relief valve that by-passes the excess flow of gas when it is warm. As the refrigerator cools down the gas in the cold end becomes denser so the high and low pressure of the gas in the system drops. The pressure difference drops and as the refrigerator approaches its designed operating temperature all of the compressor flow goes through the expander and none is by-passed. As the gas pressures drop during cool down the input power also drops. In effect the heaviest load on the compressor occurs at start up when only part of the output flow is utilized.
The problem of cooling a mass down to cryogenic temperatures is different than the problem of removing heat from a mass that is cold and is subject to heat loads from conduction, radiation, and internal heat generation. Most refrigerators have been designed to keep a load cold, frequently with heat loads that vary. U.S. Pat. No. 5,386,708 is an example of a cryopump that is maintained at a constant temperature by controlling the speed of the expander. U.S. Pat. No. 7,127,901 describes a system with one compressor supplying gas to multiple cryopumps. Speed of the individual expanders is controlled to balance the heat loads on the different cryopumps. U.S. Pat. No. 4,543,794 describes controlling the pressure (temperature in two phase region) in a superconducting magnet by controlling the compressor speed. Expander and compressor speeds have also been controlled to minimize power input.
Adding gas to a system to compensate for the increase in gas density has been described in U.S. Pat. No. 4,951,471. The use of adding and removing gas in a system using a gas storage tank for the purpose of conserving power has been described in U.S. Pat. No. 6,530,237.
In general the systems described herein have input powers in the range of 5 to 15 kW but larger and smaller systems can fall within the scope of this invention. A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a high pressure to a counterflow heat exchanger, an expander that expands the gas adiabatically to a low pressure, exhausts the expanded gas (which is colder), circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor. A reciprocating expander has inlet and outlet valves to admit cold gas into the expansion space and vent colder gas to the load. U.S. Pat. No. 2,607,322 by S. C. Collins has a description of the design of an early reciprocating expansion engine that has been widely used to liquefy helium. The expansion piston in this early design is driven in a reciprocating motion by a crank mechanism connected to a fly wheel and generator/motor which can operate at variable speed. Compressor input power is typically in the range of 15 to 50 kW for the systems that have been built to date. Higher power refrigerators typically operate on the Brayton or Claude cycles using turbo-expanders.
Refrigerators drawing less than 15 kW typically operate on the GM, pulse tube, or Stirling cycles. U.S. Pat. No. 3,045,436, by W. E. Gifford and H. O. McMahon describes the GM cycle. These refrigerators use regenerator heat exchanges in which the gas flows back and forth through a packed bed, cold gas never leaving the cold end of the expander. This is in contrast to the Brayton cycle refrigerators that can distribute cold gas to a remote load. GM expanders have been built with mechanical drives, typically a Scotch Yoke mechanism, and also with pneumatic drives, such as described in U.S. Pat. No. 3,620,029. U.S. Pat. No. 5,582,017 describes controlling the speed of a GM expander having a Scotch Yoke drive as a means to minimize regeneration time of a cryopump. The speed at which the displacer moves up and down in a '029 type pneumatically driven GM cycle expander is set by an orifice which is typically fixed. This limits the range over which the speed can be varied without incurring significant losses. Applicants' application PCTUS0787409, describes a speed controller for a '029 type pneumatically driven expander with a fixed orifice that operates over a speed range of about 0.5 to 1.5 Hz but the efficiency falls off from the best orifice setting. The speed range of this expander can be increased without sacrificing efficiency by making the orifice adjustable.
The applicant for this patent recently filed an application Ser. No. 61/313,868 for a pressure balanced Brayton cycle engine that will compete with GM coolers in the 5 to 15 kW power input range. Both mechanical and pneumatic drives are included. The pneumatic drive includes an orifice to control the piston speed. This orifice can be variable so the setting can be optimized as the speed is changed.
Applications for this refrigerator system might include cooling a superconducting magnet down to about 40 K then using another means to cool it further and/or keep it cold, or cooling down a cryopanel to about 125 K and operating the refrigerator to pump water vapor. Helium would be the typical refrigerant but another gas such as Ar could be used in some applications.